This invention relates to a simplified needle clamp and needle bar assembly that can be produced at a substantially lower cost than needle clamps and assemblies currently being made. In particular, the invention relates to a needle clamp of die cast zinc with a hardening plating layer and with a needle hole precision-formed so as to accept standard sewing machine needles of any size from #9 through #18 in only the proper orientation.
Needle bar clamps are currently made by machining the entire structure from a solid piece of steel. This structure includes a main body portion in the form of a block having a tubular part that extends upwardly and surrounds the lower end of the needle bar. A tubular side lug is formed and is internally threaded to receive a thumb screw that holds the needle in place by pressure of the end of the thumb screw on the shank of the needle.
The clamp is held in place on the end of the needle bar by means of a machine screw in a threaded hole near the lower end of the needle bar. In order to hold the needle in only one orientation in the end of the needle bar, a slot is milled into the lower end of the needle bar.
A sewing machine needle of the type most commonly used has a shank in the shape of a circular cylinder with a cylindrical segment removed. The resulting flat surface is aligned in a predetermined angular position with respect to other parts of the needle and is used as a guide in orienting the needle properly in the clamp. Needles of different size all have shanks of the same diameter, approximately 0.081", or 2.060 mm. The thickness of the shank in the direction perpendicular to the flat surface depends on the size of the blade of the needle and may be as small as 0.055", or 1.400 mm., for a #9 needle to as large as 0.064", or 1.630 mm, for a #18 needle. The width of the slot is large enough to receive the shank of the largest needle of a standard size, and the bottom wall of the slot is a planar surface at an angle of 30.degree. with respect to one of the side surfaces of the slot and, therefore, 120.degree. with respect to the other side surface. The flat surface portion of the shank is forced against the first side wall of the slot, and the round part of the shank is forced against the bottom wall by the screw that holds the needle in place.
In order to make certain that the needle can fit into the slot in only one position, a transverse slot is also milled into the needle bar just above the lower extremity, and a device called a gib is fitted into this transverse slot. The outermost surface of the gib is a segment of a circular cylinder of substantially the same diameter as the needle bar. Viewed axially, the gib is approximately wedge shaped, with the point of the wedge milled away to form a partially cylindrical concave surface that fits around the round part of the shank of the needle.
Typically, the cylindrical portion of the needle clamp that fits around the lower end of the needle bar also has a transverse slot to receive part of a thread guide formed from a metal strap that has an aperture at one end through which the screw fits to hold the needle clamp in place on the needle bar. The other end of the thread guide is bent into a loop, part of which fits into the slot formed in the cylindrical part of the needle bar clamp.
U.S. Pat. No. 2,973,733 shows a needle clamp substantially as just been described except that it does not have a gib since it was intended to provide space for two needles side by side in the slot. However, without the gib, the needle, or needles, can be put into the slot facing exactly in the wrong direction. The same possibility exists in U.S. Pat. No. 3,513,793, which is a modified form of the needle clamp shown in U.S. Pat. No. 2,973,733, and in U.S. Pat. No. 3,344,761.
U.S. Pat. No. 3,277,854 shows a needle clamp with a gib, but the gib has a flat surface facing the needle, and the slot that receives the needle in the lower end of the needle bar is wide enough to allow the needle be inserted so that it faces the wrong direction.
Needle clamps, as made heretofore, have required a considerable amount of machining. Commonly, the clamps are machined out of a single piece of metal stock, and a number of repositionings of the stock are required in order to carry out all of the machining steps necessary. In addition, the needle bar, itself, must have the slot machined in it in the longitudinal direction to receive the needle and the transverse slot to receive the gib must also be machined. The gib must be machined as a separate piece and assembled carefully with the needle bar so as to be in the proper position rather than the inverted position. This assembly is a rather delicate operation, since the gib is not only small but has no projections that allow it to be held easily while the clamp is slid onto the end of the needle bar and over the gib to hold the latter in place.